Well, one possibility is that they could use their future knowledge to prevent some of the tragedies that occurred in the original timeline. For example, they might be able to save some characters who died. They could also change the relationships between different ninja clans, making the ninja world more harmonious. And perhaps they would train harder in the past to become even more powerful ninjas in the future.
Obviously, Kakashi is one of the main characters since it's about his time travel. Minato is also a central figure as he is part of Team Minato that Kakashi travels back to be with. And of course, the other members of Team Minato like Obito and Rin are likely to be important characters as well.
Kakashi's abilities would likely be enhanced in the past. He has all the knowledge and skills he's gained in the future. So, his combat skills would be more refined. His Sharingan would be even more powerful as he would know better how to use it. He might also be able to teach his younger self some new jutsu that he learned over the years.
Their relationships would likely become stronger. They would have to rely on each other more in the unfamiliar past environment. For example, Naruto might look up to Kakashi even more as he sees how much Kakashi has grown and changed. Sasuke might be more open to the idea of teamwork as they face challenges together in the past.
A really cool plot might be that Kakashi time travels and discovers a hidden conspiracy in the past that was related to Team Minato. He then has to work with his past comrades, including Minato, to uncover the truth and prevent a great disaster that he knew about from his own time.
One good plot could be Kakashi going back in time and using his future knowledge to prevent the death of his comrades in Team Minato. For example, he might be able to stop Obito from getting crushed in that rockslide. Another plot could involve him trying to change the course of the war by influencing key decisions made by his team. He could also go back and try to train himself better in the past, using the skills he's learned in the future.
Kakashi would be really interesting. He has a lot of past experiences and knowledge. In a time - travel scenario, he could use his wisdom to guide Team 7 through different historical events. He might also have some personal moments, like seeing his old friends or mentors in the past. His sharingan could also play an important role in understanding the new situations they find themselves in during the time travel.
Maybe they time travel to a point where a major battle was lost in the past. Kakashi and Team 7 could change the outcome of that battle. For example, if they go back to the battle against the Nine - Tailed Fox when it first attacked Konoha. They might be able to prevent some of the destruction and save more lives. Or they could travel to a time when a powerful ninja was wrongly exiled, and they work to clear that ninja's name and change the course of history.
In a Sakura and Kakashi time travel fanfiction, they could end up in the past during a major war. Kakashi would be a great asset with his Sharingan and combat skills. Sakura, on the other hand, could help the wounded soldiers. Their presence in the past could change the outcome of the war. For example, they might be able to save some important characters who were supposed to die. This would then lead to a completely different future when they return to their own time.
Naruto is often a common character. Since Naruto has a strong bond with Kakashi, he might be involved in Kakashi's time - traveling adventures. Sasuke could also be there. Their relationship with Kakashi in the original story makes it likely for them to show up. And of course, Guy would be a great addition. His friendly rivalry with Kakashi would add some humor and action to the time - travel scenarios.